PHYSICS	FlO
but turn to the major divisions of physics intro
duced above d) particles and their forces (II)
the properties of matter in bulk
PABT1CLES AND FORCES
The idea that matter is composed of small
particles ot atoms originated it ib true in
Classical tunes Nevertheless the modern views
need be traced back no farther than the beginning
of the nineteenth century when Dalton and his
contemporaries were studying the laws of chemical
combination. By that time the distinctions
between elements compounds and mixtures were
already made Compounds and mixtures are
substances which can be separated into smaller
amounts of chemically distinguishable constitu
ents Elements (fee N34) cannot be bo divided
In a nurture the components may be mixed in
any proportion and sorted out again by non
chemical means In a compound the elements
jje combined in fixed proportions by weight
This last fact gives the clue to atomic theory
Dalton and Atomic Theory
Dalton pointed out that the fixed combining
weights of elements could easily be explained if
the elements consisted of atoms which combined
in simple numerical ratios eg 1 atom of element
A with one of B or one of B with two of C and
ao on For instance 35 5 g of chlorine combine
with 33 0 b of sodium to make 58 5 g of ordinary
salt If we assume one atom of chlorine links
with one of sodium then the atoms themselves
must have weights in the ratio 35 5 to 23 0
J?his turns out to be consistent with the com
bming weights of chlorine and sodium in all other
compounds hi which they both take part Some
times two elementi combine in several diffeient
proportions by weight But this is easily ex
plained by assuming that the atoms link up in
different numbers e o one iron atom with one
ox* gen or two irom with three oxygens or three
irons with four oxjgeus JChen the three different
combining proportions arise from the three cllf
ferent numbers of atoms imng in each case tlie
tame tatio ofotygen al&m ueighi to iron atom ueight
Atomic Weight
Over the century and a half since Dalton these
ideas have been repeitedly tested by chemical
experiments No one now doubts that every
chemical element has atoms of characteristic
weight By convention the number 12 0000 is
ascribed to carbon and called its atomic weight
The atomic weights of other atoms are expressed
by giving their ratio to that of carbon o g hydro
gen 1008 iron 55 85 These numbers are only
ratios the real vi eight of one single oxygen atom
is 2 7 x HT" s
3 3 Thomson and the Electron.
Matter is electrically uncharged in its normal
state but there exist many well known ways of
producing electric charges and currents—rubbing
amber or rotating dynamos for example It is
therefore necessary to have some theory of electn
city linked to the theory of matter The funda
mental experiment in this field was made by J J
Thomson when in 1897 he discxnered the electron.
If you take two metal electrodes sealed inside a
glass vessel, and if the air is suitably pumped out
and a high voltage applied to the electrodes then
the negative one emits a radiation which causes
the walls of the tube to glow The rays are called
catliode rays The discovery of the electron was
essentially a clarification of the nature of cathode
rays Thomson showed, that they were streams of
particles with mass and negative electric charge
and a general behaviour unlike any other atomic
particle known at that time The importance of
this discovery for the world of science cannot be
overestimated and its technical progeny are in
every home and factory hi radio valves television
tubes and other devices
Eutlierford-Bohr Atom.
Since the electrons emerge from matter they
are presumably parts of atoms The relation be

THE WORLD  OP SCIENCE
tween the negative electrons and the positively
chaiged constituents of matter -was elucidated by
the gieat experimenter Rutherford and the great
theoretician Bohr Their work just before the
First \\orld "U ar showed that the positive charge
together with almost all the mass is concentrated
m the central core or nucleus of the atom about
which the very light weight electrons revolve
The diameter of an atom is about 10~8 cm roughly
one three hundred millionth part of an inch The
central nucleus has a diameter about 10 000 times
smaller still Ihe nucleub and the electrons hold
together because of the electric attraction between
them
At this stage work could and did go on separate
ly along several different lines
(1) Electrons could be studied on their own
Nowadays the handling of beams of electrons
of all sizes and intensities has become a
major industry
(ii) The nucleus could be treated as a
special problem and this Jed to the mid
century flowering of nuclear physics to the
atomic bomb and to nuclear power
(ill) The behaviour of electrons in the atom
could be analysed this is the great domain
of atomic physics which spreads into many
other sciences as well
Volumes have been written about each of these
three fields but we can spare only a few hues for
each
The Electron
Electrons are expelled from solids by light
heat electric fields and other influences It has
therefore been possible to study beams of electrons
on their own in vacua Electrons inside matter
either as constituents or temporarily in transit
can also be observed by their innumerable effects
These observations all show the particles to be
indistinguishable one from another all electrons
are the same wherever they come from They
have a definite mass (911 y io~ 8 g) a nega
tive electric charge a magnetic moment and a
spin (intrinsic rotatory motion) No one
has ever subdivided an electron or obtained an
electric charge smaller than that on one electron
The electronic charge is therefore used as a basic
unit of charge hi atomic physics The electron
has come to be the best known of all the funda
mental particles
The Nucleus
The early research programmes in nuclear
physics were greatly facilitated by the occurrence
in nature of certain unstable (radioactive) nuclei
which emit fast moving fragments The hitter
can be used as projectiles to aim at other nuclei
as targets the resulting impacts yield much
valuable information This technique still
dominates nuclear physics though nowadays the
projectiles are artificially accelerated by one or
other of the large costly machines designed for
the purpose
The most important early discovery was that
the nucleus consists of two types of fundamental
particle—the positively chaiged proton and the
electrically neutral neutron These two are of
nearly equal mass (about 1 800 tunes that of the
electron) and like electrons have a magnetic
moment and spin The proton charge is equal to
the electron charge though opposite in sign
Consider a moderately complex nucleus like that
of iron This usually has 30 neutrons and 23 pro
tons Its atomic •weight therefore depends on the
total number of neutrons plus protons but the total
charge depends only on the number of protons—
called the atomic number The latter is denoted
by Z while the total number of neutrons plus
protons is called mass number and denoted by M
A species of nucleus with given values of Z and
AT is called a nwdide Z is also the number of
electrons hi the atom since the atom as a whole is
electrically neutral The atomic number deter
mines the chemical nature of the atom (see below)
so that by altering the number of neutrons in a
nucleus we do not change the chemical species
It is therefore possible to find—and nowadays to